AMB Laboratories
 
The ο1 precision master clock

News

  • Nov 22, 2016: Circuit boards are now available!

Overview

ο1 ("omicron 1") is a master clock upgrade for digital audio devices such as digital-to-analog converters (DAC), analog-to-digital converters (ADC), media streamers, CD players and digital signal processors (DSP).

Many digital devices contain a simple and low-cost clock generator based on a passive quartz crystal, similar to the schematic diagram shown below. Sometimes the inverter gate is embedded within a special integrated circuit, but the crystal and two capacitors are usually separate parts. This kind of clock generator is adequate for microcontrollers or other circuits where time-domain jitter is not important. However, on digital audio devices, jitter is a serious problem that have sonic consequences (see this TNT-audio article for a good description about jitter and its effects). Phase noise is the frequency-domain representation of jitter. A jitter-prone clock will have high phase noise.



The performance of digital audio devices that employ the quartz crystal based clock generator can be improved by disabling the original generator and adding the ο1 precision master clock. While the clock generator is not the only potential source of jitter, it is an important one and often the worst offender in digital audio circuitry. Note that some DACs use a phase locked loop (PLL) to derive a recovered clock from the S/PDIF or AES/EBU input data, and has no clock generator. The ο1 cannot be used in such devices.

ο1 uses active oscillator module(s), powered by an ultra low noise voltage regulator. The circuit is simple but effective, and will produce excellent clocks if low jitter, low phase noise oscillators are used.

Some devices require a single clock frequency whereas others need two frequencies (usually a multiple of 44.1KHz and 48KHz). The ο1 can support either type of device by populating some or all of the parts. It has a single clock output. In dual-frequency applications, a control pin allows an external circuit to switch between the two frequencies. It is also designed to be adaptable to systems with 3.3V or 5V logic levels.

For more information about the design and development of the ο1, see the ο1 development forum thread.

The following are 3D renderings of the top and bottom side of the ο1 board (click to enlarge).

Features

The following are feature highlights of the ο1 precision master clock:
  • Small PCB size (1.5" x 1.0") and simple one-screw mounting, adaptable to many devices
  • Superb low jitter, low phase noise performance when appropriate oscillators are used
  • Single or dual-frequency configuration, chosen at build time
  • 3.3V or 5V output logic level, chosen at build time
  • 5-15V input DC power, can be tapped from elsewhere in the circuit
  • Ultra low noise voltage regulator (1.0µV RMS noise, >90dB PSRR) to provide the cleanest power to the oscillators
  • Ultra wideband U.FL connector and shielded cable for the clock output
  • All surface mount devices (SMD) except the PWR/SEL/GND connector, but none of the SMDs are difficult to hand solder

What you need to know

You need to determine the following in order to build and install an appropriate ο1 for your device. Some of these may require careful visual inspection, circuit measurements and/or a service manual/schematic diagram.
  1. Your digital audio device must contain a master clock generator to be substituted with the ο1. DACs using a recovered clock are not candidates for this kind of upgrade (see Overview above). If the device already has a master clock generator with active oscillators (rather than a passive quartz crystal), then it's possible to improve its performance by simply replacing the oscillators with superior units.
  2. You must find out whether your device has a single-frequency or dual-frequency master clock circuit, and what the frequencies are.
  3. You must locate a 5V-15V DC power source in your device, to be tapped and connected to the ο1 PWR and GND pins.
  4. For dual-frequency applications, there must be a frequency selection control signal in the device to be tapped and connected to ο1's SEL pin. When this pin is at logic "high", the X1 oscillator is enabled while X2 is disabled. The reverse occurs when the SEL input goes "low". You also need to determine the SEL logic level (3.3V or 5V), and populate R3 and R4 accordingly.
  5. You should check whether your device can work with a master clock with 3.3V logic level. Most 5V based systems will work fine with a 3.3V clock. If your device doesn't work this way, then you must use 5V oscillators for X1 and X2, and use the 5V output version of the U1 voltage regulator. The input supply voltage range should then be 6-15V.
  6. You need to find a good location to mount the ο1 PCB. Most often this could be accomplished by replacing an existing mounting screw with a threaded male-female standoff, and mounting the ο1 on that standoff with the original screw.
  7. Opening and modifying your device will void any warranty that may be in effect. You assume any risk of doing the modification to your device. See the disclaimer.

Schematic diagram

ο1's schematic diagram is shown below. It is a straightforward design using a ADM7150 ultra low noise voltage regulator (U1) to provide clean DC power to the oscillators. In a single frequency configuration, only one oscillator is needed. For dual frequencies, two oscillators are necessary, and a SN74LVC1G00 NAND gate (U2) is wired as an inverter to form the frequency selection logic between two oscillators. The inactive oscillator's output is set to high-impedance state. J1 is the power/ground input header, and J2 is the master clock output.

Parts list

The following is the parts list to populate one ο1 board. 24.576MHz and 22.5792MHz 3.3V oscillators are listed.
If you require different frequencies and/or 5V logic level output, then you must choose different oscillators.
The PCB accommodates 4-pad oscillators in standard 14x9mm and 7x5mm packages.

Please pay special attention to the notes associated with each part.

Qty Part Description Vendor part numbers Notes
Mouser Digi-Key Farnell
1 PCB ο1 printed circuit board AMB audio shop -
1 R1 10Ω 1% 0805 chip resistor 667-ERJ-P06F10R0V P18937CT-ND 9332421 -
1 R2 10Ω 1% 0805 chip resistor 667-ERJ-P06F10R0V P18937CT-ND 9332421 See note 1
1 R3 1KΩ 1% 0805 chip resistor 667-ERJ-P06F1001V P16058CT-ND 9332383 See notes 1 and 2
1 R4 2KΩ 1% 0805 chip resistor 667-ERJ-P06F2001V P16065CT-ND 2303584 See notes 1 and 3
4 C1, C3, C4, C5 10µF 16V X7R 1208 or 1210 chip capacitor 81-GRM32R71C106KA01L 490-1875-1-ND 2494299 -
1 C2 1µF 16V X7R 0805 chip capacitor 81-GRM40X105K16L 490-1691-1-ND 9527710 -
1 C6 0.1µF 16V X7R 0805 chip capacitor 81-GRM40X104K16D 490-1683-1-ND 1856445 -
2 C7, C8 0.1µF 16V X7R 0805 chip capacitor 81-GRM40X104K16D 490-1683-1-ND 1856445 See note 1
1 L1 Ferrite 1206 chip bead 875-HI1206T500R-10 240-2412-1-ND 2292371 -
1 U1 ADM7150ARDZ-3.3 voltage regulator (SOIC-8) 584-ADM7150ARDZ-3.3 ADM7150ARDZ-3.3-ND 2377357 See note 4
ADM7150ARDZ-5.0 voltage regulator (SOIC-8) 584-ADM7150ARDZ-5.0 ADM7150ARDZ-5.0-ND 2377358 See note 5
1 U2 SN74LVC1G00DBV NAND gate (SOT23-5) 595-SN74LVC1G00DBVR 296-9845-1-ND 1287607 See note 1
1 X1 CCHD-957-25-24.576 3.3V 14x9mm oscillator 549-CCHD-957-25-24.5 744-1457-ND - See note 4
1 X2 CCHD-957-25-22.5792 3.3V 14x9mm oscillator 549-CCHD-957-25-22.5 744-1456-ND - See notes 1 and 4
1 J1 Molex KK .254 3P header (vertical) 538-22-23-2031 (tin) or
538-22-11-2032 (gold)
WM4201-ND (tin) or
WM2701-ND (gold)
1462950 (tin) or
2112431 (gold)
-
Molex KK .254 3P header (right angle) 538-22-05-3031 (tin) or
538-22-12-2034 (gold)
WM4301-ND (tin) or
WM2712-ND (gold)
1462932 (tin) See note 7
1 J1' Molex KK .254 3P plug 538-22-01-3037 WM2001-ND 1462838 -
3 - Molex KK .254 crimp terminal 538-08-50-0114 (tin) or
538-08-55-0102 (gold)
WM1114-ND (tin) or
WM2312-ND (gold)
1462641 (tin) or
1462642 (gold)
-
1 J2 Hirose U.FL PCB-mount socket 798-U.FL-R-SMT10 H9161CT-ND 1688077 -
1 J2' Hirose U.FL shielded cable and plug 798-U.FLLP066J1A200 H125221-ND 2491378 See note 6
- - 22-24AWG stranded hookup wires (your choice of lengths and colors) See note 8

Notes:
  1. For dual-frequency configuration only.
  2. Only for 3.3V configuration but SEL logic is 5V based. Otherwise short this resistor.
  3. Only for 3.3V configuration but SEL logic is 5V based. Otherwise leave it open circuit.
  4. For 3.3V configuration only.
  5. For 5V configuration only.
  6. A 200mm (8") long, 1.32mm diameter coaxial cable is listed, terminated on one end with U.FL plug and unterminated on the other end. Other cable thicknesses, lengths and double-terminated versions are available.
  7. Right-angle pin headers, for use when there is not sufficient clearance within the chassis to accommodate vertical pin headers.
  8. For power, ground, and SEL logic wiring.

Parts selection

  • Resistors and capacitors:
    The part numbers listed above are examples only, and can be substituted with other brands/models as long as the size and ratings are the same.
  • Oscillator(s):
    Standard 14x9mm and 7x5mm foot prints are supported. The Crystek CCHD-957 series are the top performers, but the Fox Xpresso FXO-HC736R series are much less expensive alternatives. Crystek C3391 series are also very good. These are all 3.3V oscillators. Oscillators for other frequencies and voltages are available. Crystek C3391 oscillators are available from Mouser, Fox Xpresso oscillators are available from Mouser and Arrow Electronics. You can do parametric searches at the vendor websites for other usable oscillators.
  • Other parts:
    Please use the items as listed above.

The circuit board

The ο1 board is made of high quality FR-4 glass epoxy, with double copper layers, silkscreen and soldermask on both sides. The dimensions of the board is 1.5" x 1.0" (38.1mm x 25.4mm). The thickness of the board is 0.062" (1.58mm). One single screw hole is provided for flexible mounting in different applications.

Here are images of the PCB layout, including the silkscreens, and photos of an assembled ο1 board in single-frequency configuration.

Top side Bottom side

Recommended tools and supply

Due to ο1's use of surface-mount devices (SMD), you should have the following tools and supplies to help you work on the board:
  • A good soldering iron with fine tip, preferably with adjustable temperature. For example, a Weller WLC100 with ST6 tip. A more deluxe soldering station such as the Weller Weller WES51 or Hakko FX888 is nice, but not necessary.
  • Liquid flux or flux pen (e.g., this or this)
  • Thin gauge solder, such as 0.025". Your choice of 60/40 or 63/37 tin/lead. Avoid silver solder as it requires high heat. Lead-free solders also require higher heat and their durability is still in question.
  • Fine tweezer with sharp points, such as those from Wiha
  • Desoldering braid
  • Chip Quik® SMD desoldering kit (if needed).
  • Vision aid, such as magnifier lamp, loupe, magnifying eyewear (e.g., Fisherman Eyewear Flip-n-focus), etc.
  • Needle nose pliers
  • Diagonal cutter
  • Cotton Q-tips
  • 99% isopropyl alcohol or specialty electronics flux remover spray
  • Multimeter with sharp probes

Circuit board assembly instructions

  1. The following steps assume that you have done everything in the "What you need to know" section above, and have the information to appropriately build and install the ο1 circuit in your device.
  2. If you're new at soldering surface-mount devices, please view the following video, an excellent tutorial.
    Surface Mount Soldering 101
    by Curious Inventor
  3. Clean both sides of the blank ο1 board with paper towel and isopropyl alcohol or electronics flux remover, then solder the components to the board. Work on the bottom side of the board first, then the top side.
  4. Make sure the correct part goes into each position on the circuit board.
  5. For every part except the 3P pin header, apply liquid flux to the board solder pads, and place the part over the pads. Use the tweezers to pick and nudge the part until it is centered and aligned perfectly. Be sure that the pin orientation or polarity of U1, U2, X1 and X2 is correct. Most chips have a dot marking next to its pin 1. For SOIC-8, the top edge of the package where pin 1 is located is usually beveled. That edge is represented as an extra white line on the board silkscreen.
  6. On the bottom side, it is recommended that you solder the integrated circuits first, then the passive parts. On the top side, solder the passive parts first before the oscillator(s) and 3P pin header.
  7. Press the tweezer tip on the top of a part to keep it from shifting while soldering. Apply only a tiny amount of solder to the tip of your iron, and tack down one corner pin of the chip, or one side of a 2-pad part. It helps to use a "wiping" motion of the tip on the pin and pad. If necessary, make small adjustments while heating that pad and pin again. If all is well, do the pin on the opposite side (diagonally-opposite side on parts with more than 2 pins). Then, do the remaining pins, if applicable, one at a time, and reflow any pin that needs a bit of touch-up. If necessary, use the desoldering braid to remove any excess solder, and be sure there are no solder bridges between adjacent pins or pads.
  8. Use your multimeter and check the resistance between each pin of the part and the pad to make sure you don't have a cold joint (i.e., your meter should read close to zero ohms). Also, on parts with more than 2 pins, check continuity between adjacent pins for solder bridges (meter should read infinity.
  9. Repeat the above procedure for each of the SMD components.
  10. Mount and solder J1 (3P pin Molex KK header) last.
  11. Inspect your work carefully with a magnifier.
  12. Clean up the solder flux residue on both sides of the board with isopropyl alcohol (or electronics flux remover) and a brush.

Installation instructions

Details vary depending on the device you're installing the ο1 into, but it consists of two general operations - Disabling the existing clock, and installing and wiring the ο1.
  • Disabling the existing clock

    1. You must disable the original quartz crystal clock generator by removing the crystal itself, the parallel resistor if it exists, and the two capacitors to ground. The left side capacitor pads should be shorted together so that the inverter gate input does not float. All these are marked in red color in the following diagram:



    2. The output resistor (before the MCLK label) should also be removed, if it exists. This disconnects the original master clock generator from the rest of the circuit. If your circuit does not have such a resistor, you should cut the PCB trace at that point. The location indicated by "A" is where the clock output from the ο1 should be tapped in (see below).

  • Installing and wiring the ο1

    1. Find a good location to mount the ο1 board. The most convenient way would be to use a male-female standoff in an existing screw hole, and mount the ο1 on the standoff with the original screw.
    2. The PWR/SEL/GND wire lengths and the U.FL shielded cable length are dependent on where you mount the ο1, and where in the device these wires are to be connected.
    3. Strip the PWR/SEL/GND wires, crimp (and solder) Molex crimp pins on one end of these wires and insert them into the appropriate slots of the Molex 3P plug housing. Solder the other end to the locations in the device previously determined.
    4. Strip the outer sleeve of the U.FL shielded cable and separate the ground shield from the center wire. Twist the ground shield end tightly and tin it with solder. Also tin the stripped end of the center wire. Tack solder the center wire to point "A" as shown in the diagram above, and solder the shield to a nearby ground.
    5. Reinstall any removed boards and mount the ο1 PCB, connect the PWR/SEL/GND Molex plug to J1. Double check for correct pin assignment.
    6. Connect the terminated end of the U.FL shielded cable to J2 on ο1. Note that the U.FL connector is designed to be plugged and unplugged for only a few times.
    7. Double check for errors again.
For a specific installation example (3.3V, single-frequency configuration master clock upgrade for a Behringer DCX2496 DSP loudspeaker management system), see this forum post.

Initial check

  • With power off, use your multimeter in Ω mode to measure the resistance between VCC and GND to make sure there is no short circuit.
  • Power up the device, and use the multimeter in DC V mode to check the voltage at the V+ test pad on the bottom of the ο1 board. This test pad is also accessible on the top side of the board, but is unlabeled. Use the oscillator body as the ground reference. The voltage you read should be 3.3V or 5V, depending on the U1 voltage regulator you selected.
  • Connect your device to the system and play some music of various sample rates. This verifies that everything is working correctly.
  • Optional: You can measure the clock waveform with a 50MHz or higher oscilloscope. Be aware that an ordinary oscilloscope probe ground wire usually has enough inductance to distort the displayed waveform. Thus what you see on the scope is not necessarily how it actually looks like on the board.

Clock waveform

These are the ο1 clock output waveforms, as captured on a Tektronix TDS2014B digital storage oscilloscope, with a special coil ground contact installed on the test probe. The ο1 tested here is single-frequency, 3.3V configuration, with a Crystek CCHD-957-20-24.576 oscillator. In the second oscillogram, the small overshoot at the leading and trailing edges of the clock is due to the unterminated transmission line, and is not a problem.

At the U.FL output connector At the end of the U.FL shielded cable (unterminated)

Additional information

  • Refer to the oscillator datasheet for information about its jitter or phase noise performance (if it's specified).
  • Visit the AMB DIY audio forum for support.

Copyright © Ti Kan
All rights reserved. Commercial use of this design is prohibited without prior permission.